Dynamic galvanometer



[ A A A June 6, 1950 E. w. KELLoGG 2,510,585

" Y DYNAMIC GALvANouETER Filed Aug. 21,1945v 2 sheets-sheet 2 IN VENTOR W/f//M ATTORNEY l Patented June 6, 1950 2,510,585 DYNAMIC GALvANoME'rER Edward W. Kellogg, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application August 21, 1945, Serial No. 811,890

Claims. l

I'his invention relates to light modulators, and particularly to modulators of the galvanometer or oscillograph type used in sound recording.

Sound recording galvanometers are wel1 known. One standard method of commercially record-- ing sound employs a galvanometer wherein the light beam is projected on a mirror thereof, the mirror vibrating the light beam in accordance with the instantaneous amplitude values of the sound waves, or in accordance with both the instantaneous values and the envelope value. The presentl invention is directed to a novel type of galvanometer construction which has several advantages over the present known types of structures. Galvanometers at present or previously used for sound recording are of two types: (1) oscillograph type galvanometers, sometimes called bifllar suspension" galvanometers, and (2) magnetic armature galvanometers, in which the coil itself is stationary. The most successful galv'an'ometers are of the magnetic armature type, but since it is diflicult to so design the armatures that they willrctate through any appreciable angle, the mirror is not carried directly on the armature but linked thereto in such a way that a small translation of the armature produces substantial rotation of the mirror. Such a galvanometer construction is disclosed and claimedin Dimmick U. S. Patent No. 1,936,833 of November 28, 1933. In other words, the rotary motion is "geared up between the armature and themirror. This gearing up involves some memirror is attached by suitable. cement to both ribbons, and when one conductor moves forward and the other backward, the mirror rotates.

It is one of the principles of design of galvanometers for sound recording that in order that they may respond in the manner required, the fundamental resonance or natural frequency of the galvanometer must be near the upper end of the range of frequencies which is to be recorded. For high quality speech, this is usually taken as 8000 to 9000 cycles per second. This high resonance frequency means that the stinchanical problems, and in all magnetic armature galvanometers. some distortion is caused by hysteresis in the ferromagnetic material of the armature and pole pieces. To offset these disad- -vantages, the magnetic armature galvanometers ca swing much larger mirrors than hitherto ayailable oscillograph galvanometers of comparjlaible sensitivity and frequency response. Galvaf'nometers of the moving conductor type lend 'themselves to larger angles of rotation of the mirrerbwithout'resort to the expedient of gearing una for the reason that conductors carrying cur.. rent in opposite directions can be located close together in a single magnetic field, and, therefore, move in opposite directions and cause sublstantial rotation. However, the only galvanometers hitherto available of the moving conductor type, which are suitable for operation at the high frequencies necessary for sound recording, are those-of the oscillograph type. In these, two metal ribbons are stretched close together betw'een supports approximately V2v inch apart, and a magnet produces a strong transverse field. A

ness of the supporting and centering members must be high in relation to the eetive mass of the moving parts. In the oscillograph type galvanometers, the stiffness is obtained by putting the ribbons under tension, and low mass is obtained bymaking the ribbons thin and the mirror small as well as thin. In particular, the moment of inertia of the mirror must be low about its axis of rotation, which means it is a narrow mirror. The fact that the mirror is between the magnet poles also makes it practically necessary to keep it narrow so vthat the gap may be short for the sake of having an intense magnetic field. Typical dimensions are .017" x .060" by .005" thick for the mirror, and ribbons .001" x .006" in a magnetic gap about slr long.

In order to tune the galvanometer up to 8000 or 9000 cycles, the ribbons must be stretched up to near their breaking point, and they must be made of material which is very tough as well as having good conductivity. Damping, to prevent excessive amplitudes at resonance, is obtained by submerging the entire suspension in a clear oil of suitable viscosity. This oil, in which the ribbons moveas paddles, adds considerable effective mass in addition to the desired damping. In a galvanometer intended to work at frequencies as high as required for sound, the angle through which the mirror can be rotated depends on the driving moment divided by the moment of inertia of the moving Darts, which includes that of the conductors, the mirror, and any other incidental moving elements. The current carrying capacity of theconductors goes up approximately with their mass. Hence, the useful torque goes up with the product of conductor ymass by the mean separation of the conductors. But the moment of inertia of the conductors is propor-f tional totheir mass multiplied by the square of their mean separation. Tins relationship makes it desirable to have the conductors close together,

but it may be shown that the conductors should be heavy enough -and farA enough apart so that their moment of inertia is equal to that of the inactive load which they must swing, which includes the mirror and any necessary structural elements. The construction of oscillograph galvanometers is such as to practically dictate the use of very thin conductors, and only a single turn or pair of active conductors. The condition of approximate equality of moments o! inertia is attained at the cost of using a very small mirror, which correspondingly limits the amount of light which can be modulated.

In the galvanometer which is the subject of this invention, I employ a mirror having at least ten times the area of those comonly used in galvanometers of the oscillograph type, and employ enough conductor material to bring the moment of inertia of the active conductor up to at least es high a value as that of the mirror plus all other inactive moving elements, such as the structure on which the coil and mirror are supported, while, at the same time, keeping the conductors carrying current in opposite directions close together. The material employed for the conductors is preferably aluminum, since this has the highest ratio of conductivity to mass of all available metals. The desired moment of inertia is obtained by making the coil long, rather than large in other dimensions, and the structure. comprising coil and mirror, is sufficiently rigid so that electromagnetic forces, acting on all parte of the coil, are fully eiiective 'at the mirror at the highest frequencies which it is desired to record. An advantage of the long coil is that while the 4increase in conductor length adds tov the resistance, and hence, to the heat for a given current, the heat dissipating surface is increased in the same proportion.

The principal object of the invention, therefore, is to facilitate the modulation of light by a vibratory deiiector.

Another object of the invention is to provide a galvanometer combining the advantages of large permissible deiiection and low distortion, characteristic of the moving conductor type, with a large mirror which is at present available only in the magnetic armature type.

A further object of the invention is to modulate more light than is at present possible.

A still further object of the invention is to provide a galvanometer having improved damping characteristics.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner of its organization and the` mode of its operation will be better understood by referring to the following description read in commotion with the accompanying drawings forming a part hereof, in which:

Fig. l is a plan cross sectional view of the galvanometer embodying the invention.

Fig. 2 is an elevational cross sectional view of the galvanometer construction involving the invention.

Fig. 3 is a cross sectional view showing the position of the armature windings.

Fig. 4 is a cross sectional view illustrating the direction of motion of certain conductors which are in a position departing materially from the optimum.

Fig. 5 is a cross sectional view showing a modication of the invention suited to galvanometers in which the coil is oil-immersed for purposes of cooling the conductors and for damping, and

Figs. 6 and 7 are plan and elevational views,

respectively. oi' another modiidcation of the invention in which the galvanometer has a singleturn coil.

Referring now to the drawings, in which the same numerals identify like elements, the galvanometer is provided with a magnet having pole pieces 5 and 6, pole piece 5 being a south pole;I and pole piece 8, a. north pole. Positioned in the air gap formed between the pole pieces 5 and i is a supporting bar 9 having a substantially square cross section except near the point of anchorage, the bar 8 being preferably of light non-magnetic material. such as an aluminum or magnesium alloy. It may be made of soft iron to improve the concentration of the magnetic flux, but the use of iron increases the mass as compared with aluminum, makes it more diiiicult to center the coil in the gap, and brings in the possibility of some distortion due to hysteresis, although this distortion can be made extremely small. It is preferable, in general, to get the desired magnetic field by using a powerful eld magnet. It is not essential that the bar 8 be of metal. Light weight and adequate strength and rigidity are the requirements. Insulation problems are simplied by making it of non-conducting material.

mounted with its center of gravity substantiallyV coincident with the axis of the bar. The bar passes through several bushings or diaphragms l5, I6, and l1 of exible material, such as rubber, which serve as bearings to permit rotationthrough a limited angle, and effectively prevent lateral displacement or vibration.

Damping, to prevent excessive amplitude at resonance, is provided by a torsional wave line I9 of rubber. or similar material. Such a line.

is described in a paper entitled Methods of High Quality Recording and Reproduction of Music and Speech, Based on Telephone Research, by J. P. Maxfield and H. C. Harrison, in the Transactions of the A. I. E. E., volume XLV, February, 1926, page 334. The line is a cylinder or rod of material having a very low elastic modulus of shear and fairly large internal mechanical losses, but preferably of moderately high specific gravity. When a twist is applied to the end of such a rod, the movement is transmitted along the rod as a wave, and owing to the above-mentioned properties of the material, the rate of propagation is very slow as compared with that in other solids, and hence, at a given audio frequency,

the waves are very short, and a line a few inches long may, at audio'frequencies, be a number ofv Wave lengths long. At the same time, the waves are attenuated or weakened very much in traveling the length of the line. When a wave reaches the far end of the line, whether the end is free or constrained, it is reflected and returns toward the driven end. However, if the line is long enough and the rate of attenuation high, the wave is of negligible magnitude when it gets back to the starting end. Under these conditions, the line reacts on the member that imparts vibratory motion -to it, substantially like a pure resistance and not like a spring or a mass load. The magnitude of the resistance in dyne cms./radians per second is equal to IVZ in which same quality of material is desirable and the piece is of such dimensions that it is antiresonant, and whips at the resonance fre-v quency. An example of an anti-resonant damper is a line of the type described above, but just one-quarter wave length long with the end free, or a half wave length long with the end anchored. In general, the long line is to be recommended, since it clamps at all frequencies.

The actuating coil is formed of conductor 2l which is wound ,on opposite sides of the bar 8, as shown in Figs. 1, 2, and 3, the coil being preferably of aluminum wire. This long, narrow -'coil shape provides a large ratio Iof active to inactive conductor. Referring to Fig. 4, the conductors at the corners produce the same amount of driving torque as those close to the median plane M, since the forces on all are perpendicular to the magnetic field and all have the same moment arm b/2; but the corner conductors, as illustrated by the arrows, m'ove farther for a given angle of rotation than those near the middle. Hence, it is desirable to keep c small. preferably substantially less than b, if b is the separation of the outermost layers.

l Assuming that c is small compared with b, a calculation may be made to indicate whether the entire available space should be filled with active conductors, or whether it is better to devote a substantial fraction of the space to the bar 8 which is electromagnetically inactive. If we assumed the current density to be the same throughout the winding space, whether all or only part of the space is devoted to active conductors, there would be some gain in torque from filling up the entire space with current carrying conductors, provided adequate rigidity could be achieved in such a structure. However, a more reasonable assumption is that the total wattage should be kept the same, since the heat dissipating ability of the coil depends primarily on its outside surface. If it is assumed that the bar has the same density as the winding, which is reasonable, since the bar would normally be ranges, swing extra large mirrors, or vibrate through large amplitudes. For such applications, liquid cooling may be desirable, and the liquid may provide some or all of the desiredv damping. Immersing any part of the moving system in liquid will add some inertia which reduces sensitivity. This added inertia is mini-y mized if the mirror is kept outside the liquid, for on account of its shape, it would, if lm mersed.

of aluminum alloy and the conductors of, alumis act as a paddle, and by giving the immersed por` tion of the moving structure a substantially cylindrical shape. Such a construction is shown in Fig. 5. The bar andwinding would normally 5 be a rectangle about three times as long as it is wide. This is built out to a substantially circular cross section by attaching two strips 22 of light material having cross sections in the shape of circular segments. If these strips are of porlo ous material, their surfaces should be given an impermeable 'coating so that the liquid will not soak in and increase their mass. A preferred construction is to make them of very thin walled aluminum tubing, flattened to the desired shape 15 and with their ends hermetically sealed. The

lower the viscosity of the liquid, the less ofit will be carried back and forth with the tangentially m ling surfaces, and thus, the less will be the mass added by the liquid. A liquid of moderately 20 low viscosity may not give as much damping as is desired if the clearance between the moving l surface and the nearest stationary surface is large. Hence, it may be desirable to completely surround the coil with a concentric stationary 25 surface leaving as small clearance as is mechanically practical. This may be accomplished by the Puse of ytwo appropriately shaped blocks 23, which, with the pole pieces 5 and 6, form a cirto carry away the heat. I do not recommend that the oil or other liquid be used as the sole source of damping, for it is likely to be considerably affected by temperature. The balance of the damping may be provided with a wave transmission line I9, and the liquid may serve primarily for cooling.

Keeping the mirror outside the liquid-filled chamber may be readily arranged by passing the bar 8 through a bearing diaphragm I6 which is made of rubber or similar material and issealed to the bar yand to the walls of the liquid compartment.

Although it is, in general,- advantageous to employ a multiturn coil`and work with small currents, a single-turn coill offers some advantages in carrying away heat, in the first place because the bare conductor canv stand more heat and radiate it more readily, and secondly, be-

high and a considerable wattage may be carried longitudinally of the conductor-sto the anchor terminals with only a few degrees of temperature difference. Figs. 6 and '1 show the construction of .a single-turn galvanometer. The

from a single'pie'ce of aluminum` or high `conductivity aluminum alloy. In order to give the desired torsional rigidity throughout the length `of the active coil, an insulating spacer 24 is inserted between the conductors, to which it must be very firmly attachedv by cement, supplemented...

if necessary, by two wrappings of thread 21. On

5 the other hand, between the end of spacer 2| sirable to materially reduce the cross section of the conductors at any point, but they may be flattened to increase their flexibility and given sufficient length to tune the system to the desired natural frequency. The terminals 26 should be massive enough and have suflicient area to carry away and radiate all of the heat generated,

and they may be eitherfone piece with the concular wall. The blocks should be large enough cause the heat conductivity of aluminum is very coil has an extension 25 which corresponds t0 a part of bar 8 in Figs. l1 and 2 and is formed 7 ductors, or connected to the same through a large area contact to insure good heat transfer.

I claim:

1. A galvanometer comprising means for obtaining a magnetic field, an armature support positioned in said eld, an actuating coil wound on said support, means for flexibly mounting said armature support for rotation at a plurality of bearing points, said mounting preventing lateral displacement of said armature, a mirror mounted on said armature support externally of said magnetic field, and a line damper mounted on the end of said armature support.

2. A galvanometer comprising means for obtaining a magnetic field, anl armature support positioned in said field, an'actuating coil wound on said support, means for flexibly mounting said armature support for rotation at a plurality of bearing points, said mounting preventing lateral displacement of said armature, a mirror mounted on said armature support externally of Asaid magnetic field, one end of said armature support being anchored and a free end damper line on the other end of said armature support, one of said bearing points being positioned intermediate said coil and said anchorage.

3. A galvanometer comprising means for obtaining a magnetic iield, an armature support positioned in said field, an actuating coil wound on said support, means for flexibly mounting said armature support for rotation at a plurality of bearing points, said mounting preventing lateral displacement of said armature, and a mirror mounted on'said armature support externally of said magnetic field, an anchorage being provided for one end of said support, said support having a substantially square cross section except for the portion thereof between said coil and'said anchorage, said coil being wound in layers on opposite sides of said support, said coil and support having a cross-sectional area substantially three times as long as it is wide.

4. A galvanometer construction comprising means for producing a magnetic iield, a substantially square'shaped bar extending through said wound on said bar, means for anchoring one of said extended ends of said bar, said bar.being narrowed between said coil and said anchorage, a mirror mounted'on the other extended end of said bar in a manner whereby the center of gravity of said mirror is coincident with the axis of the main portion of said bar, and means for resiliently supporting said bar in said magnetic field, one o'f said means being positioned between said coil and said anchorage.

5. A galvanometer construction in accordance with claim 4, in which said winding is wound in layers on opposite sides of said bar. y

6. A galvanometer construction in accordance with claim 4, in which a free end damper line is provided and attached to the free end of said bar.

7,'A galvanometer comprisingl a magnetic structure -having spaced parallel pole pieces defining an air gap, an armature in said air gap,

said armature including a support and a conductor wound on opposite sides of said support, a damper mounted on the end of said support,

means for flexibly mounting said armature at a said coil may be immersed in said liquid, said the cross-sectional area of said armature being substantially three to one.

8. A galvanometer in accordance with claim 7, in which a mirror is attached to said support outside of said air gap with its center of gravity substantially coincident with the axis of said support, said support being deformed into a U 'at the point of attachment of said mirror thereto.

9. A galvanometer comprising a magnet and pole pieces providing a region of strong magnetic field, a coil of aluminum conductor located in said eld, a support for said conductor, the crosssectional area of said conductor and said support being substantially three times as long as it is wide, a mirror located external to said field, connecting means whereby rotation of said coil is imparted to said mirror, and mounting means for permitting rotation and preventing lateral displacement of said coil, said coil and connecttions of said conductor on the two sides of said coil being of the order of or less.

10. A galvanometer comprising a magnet and pole pieces providing a region of strong magnetic field, a coil of` aluminum conductor located in said field, asupport for said conductor, the crosssectional area of said conductor and support being three times as long as it is wide, a mirror located external to said field, connecting means whereby rotation of said coil is imparted to said mirror, and mounting means for permitting rotation and preventing lateral displacement of said coil', said coil and connecting means having such torsional rigidity in relation to the moments of inertia-of said coil Vand said mirror that when current of 'the highest important audio frequency is sent through said coil, the structure comprising said coil, connecting means, and mirror, will vibrate with substantially no difference in angular amplitude or phase, the mean separation bequency at least as high as the geometrical mean I of the range of audio frequencies to which the galvanometer is intended to respond.

12. A galvanometer in accordance with claim 10, in which the entire length of the sides of the coil is within the region'of strong magnetic eld, and in which the moment of inertia of the side conductors of the coil is -of the same order of .magnitude as that of the mirror plus that of all other electromagnetically inactive material in the rotating structure.

13. A galvanometer in accordance with claim 9. in which is provided a casing for said'magnet, coil, and connecting means, and a diaphragm of flexible material is provided surrounding said connecting means to form a rotationally flexible seal, liquid being provided in said casing whereby mirror being without said liquid.

14. A galvanometer in accordance with claim mounting points, the ratio of length-to-width of 15 10, in which is provided a casing for said magnet,

coil, and connecting means, and a diaphragm of exible material is provided surrounding said connecting means to form a rotationally flexible seal, liquid being provided in said casing whereby said coil may be immersed in said liquid, said 5 mirror being without said liquid, the immersed portion of the rotatable structure being built out to a cylindrical shape.

15. A galvanometer in accordance with claim 10, in which the coil and mirror structure is provided with a plurality of rotationally flexible supports, said supports being designed with suilicient rotational stiffness to cause the galvanometer to have a fundamental natural resonance at a frequency at least as high as the geometrical mean of the range of audio frequencies to which the galvanometer is intended to respond, and damping means are provided substantially adjacent to the mirror.

EDWARD W. KELLOGG.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,800,601 Centano Apr. 14, 1931 1,820,785 Dimmick Aug. 25, 1931 1,943,112 Curtis Jan. 9, 1934 10 2,183,934 Heiland Dec. 19, 1939 2,313,129 Dohan Mar. 9, 1943 2,389,081 Redmond Nov. 13, 1945 FOREIGN PATENTS 15 Number Country Date 518,759 Great Britain Mar. 6, 1940 

